Spend Game by Jonathan Gash
When Lovejoy witnesses a car crash that turns out to be a murder - with one of his oldest antique-dealer friends the victim - he sets out on a trail of revenge that leaves him pondering several bewildering questions. Why did his friend buy up a load of junk furniture? What did he want with an old doctor's bag? Why was his friend killed? Who was trying to kill Lovejoy and - most perplexing of all - what the hell is he doing potholing through underground tunnels dodging armed hit men?